What Causes Brown Foul Smelling Discharge After Menstruation?

Hi, may I ai had unprotected sex with my boyfriend and took postinor 2 within the stipulated 72hrs.6days after i started my periods which natuarlly would have started 8 days after that day didnt worry as i thought it was the morning pill taking effect but the blood was dark and had a bad smell after i finished i started to have a brown smelling discharge.what could be the cause?please help